Control of power factor at the input of a power circuit, powered off of an AC power line typically designated as an off line switcher (OLS), is critical to both the integrity of the AC power line as well as the efficient operation and transient response of the power circuit itself. In theory the power factor can attain a unity value by forcing the input current waveform to conform exactly to a sinusoidal waveform in phase with the fundamental of the sinusoidal voltage waveform input. Many techniques have been advanced to achieve this current waveform control. Some of the earlier techniques use passive networks with reactive components to shape the input current waveform. As power factor and other operating requirements of the power supply become more demanding the trend has been toward the use of active power factor control networks to control the input current waveform.
Active power factor control networks typically sense input and output signal parameters of the power circuit and utilize a rectifier followed by a boost, buck, buck-boost, SEPIC or similar power trains connected between the AC line and the power circuit to enhance the power factor. The boost power train includes a power switch selectively switched or pulse width modulated in response to these signal parameters to force the input current to conform to a desired or programmed current waveform. In a particular illustrative arrangement disclosed in U.S. Pat. No. 4,412,277 a rectified input AC voltage waveform is multiplied with an error voltage representing the deviation of the output voltage from a regulated value. The resulting control signal is scaled to provide a programmed AC current waveform i.sub.p. This waveform is used to control the modulation of a pulse driving the power switch of the boost power train to provide the desired input current waveform and hence advance the power factor value more closely to a unity value.
An improved power factor control arrangement disclosed in U.S. Pat. No. 4,677,366 uses an instantaneous rms value of the input AC voltage as a control variable to provide a suitable transient response to changes in the amplitude of the waveform of the AC line voltage. This control arrangement includes a feed-forward control, added to accommodate rapid changes occurring in the rms value of the input AC voltage. This feed-forward arrangement scales the programmed current i.sub.p inversely by the square of the rms input voltage.
A problem with these existing arrangements is the effects of ripple voltage due to rectification and other causes superimposed on the sensed voltage waveforms. This ripple voltage in the sensed signals is a spurious signal which is superimposed on the error voltages used to control the boost converter. This prevents an accurate determination of the waveform of the programmed current i.sub.p, and creates undesirable side effects in the operation of the control circuitry. Present techniques advanced to deal with this ripple voltage slow the response time of the power factor control circuitry.
Another problem with existing arrangements to enhance power factor is the slow response time of output voltage regulation control loops to output load transients. Existing arrangements to address this problem include output power as a feed-forward variable in the feed-forward control loop controlling the power switch of the power train. A key variable in the effect of the output power on the control process is the energy stored in the output capacitor of the power train.
A controller designed to accurately accommodate changes in output power has been implemented as a digital controller and is disclosed in the disclosure entitled "A Digital Controller for a Unity Power Factor Controller" Mitwalli et al, Workshop on Computers in Power Electronics, Berkley, Calif., August, 1992. This controller is based on modeling instantaneous power flows and is based on knowledge of the value of the power train's output capacitance. It additionally requires complex real time calculations to achieve satisfactory operation.